Heat exchangers for gases and liquids--particularly counterflow heat exchangers--have long been known and used for a variety of purposes. Some of the notable applications have included: placing a heat exchanger on a building's exhaust outlet so that heated exhaust air which normally must be vented from a building may be used to preheat incoming fresh air to minimize the energy needed to heat that incoming air to a desired temperature, use in commercial water heaters, attachments for stove and furnace flues, and many industrial chemical engineering processes.
The principles of heat transfer dictate that as two fluids (liquids or gases) at different temperatures are passed through a heat exchanger, a differential temperature gradient will be established and each fluid will tend to approach (but not reach) an equilibrium temperature. That equilibrium temperature is a function of the density and molecular structure of each fluid, the coefficient of heat transfer within the exchange gradient, the ratio of the total volume of each fluid to the surface area of the exchange gradient, and the length of time that each fluid is in contact with the transfer gradient.
In most applications, the fluids used in the heat exchanger are controlled by the function of that heat exchanger--usually ambient and exhaust air, or water. The coefficient of heat transfer for the gradient is a physical property of the material used, which is again dictated by the function of the heat exchanger, as well as economic considerations and the available manufacturing methods. Sheet metal, ductwork, metal pipe or firewall tubing, and glass are commonly used materials.
While the total volume of each fluid within a given heat exchanger and the surface area of the gradient are somewhat within the designer's control, there again are economic and practical constraints limiting the total size of the heat exchange apparatus and the surface area of the exchange gradient. Often, piping must be of a particular minimum diameter or wall thickness so as not to impede the flow of fluids through the heat exchanger, or to comply with local safety and construction standards, and the total volume of the heat exchanger is limited by the area in which it will eventually be used.
Furthermore, a heat exchanger designer faces the theoretical task of balancing the reciprocal relation between the cross-sectional size of the channels which carry each fluid and the resulting speed at which that fluid will travel through the channel according to Bernoulli's equation. Decreasing the cross-section of a channel so that a greater length of channel may be placed within a heat exchanger (to increase the surface area of the gradient or lengthen the time that the fluid flows within the exchanger) has a detrimental effect of increasing the flow rate for that fluid and decreasing the time which that fluid is in contact with the gradient, thus reducing the efficiency of the heat exchanger.
Solutions to maximize the heat exchange characteristics are the subject of a variety of multiple linear differential equations relating each variable, such as the volume or temperature of each fluid, the surface area and coefficient of heat transfer of the gradient, and the length of time each fluid is in contact with the gradient.
Because these relationships are generally linear, one result reached is that the temperature of the transfer gradient at any one point can achieve equilibrium (assuming a relatively constant flow rate for each fluid) although the fluids themselves do not reach thermal equilibrium. Consequently, within the confines of the physical dimensions for a particular unit volume heat exchanger and its component channels, it is a general rule that increasing the longitudinal path for each substance along the transfer gradient will increase the efficiency of that gradient, and have the greatest effect on bringing the total volume of each substance closer to a higher thermal equilibrium, thereby also increasing the efficiency of the heat exchanger. Distinquishing the efficiency of the gradient from the efficiency of the heat exchanger itself is one key to recognizing why the heat exchanger of this invention can achieve such a drastic overall increase in heating efficiency.
When the aim of the heat exchanger is to transfer the maximum amount of thermal energy in one direction, the general rule will apply so long as the flow rate of the fluid to be heated is not increased to a point where the total time during which the total volume of that fluid is in contact with the heat exchange gradient is diminished.
As such, one problem facing heat exchanger designers has been to find a configuration which increases the longitudinal path of each fluid along the transfer gradient without unduly decreasing the time each fluid contacts that transfer gradient.
A second problem facing the designer is to guarantee that each fluid (particularly that to be heated) is thoroughly agitated as it passes through the heat exchanger, to ensure complete and uniform distribution of the thermal energy. Convection layers or isothermal pockets (small volumes of fluid closer to the equilibrium temperature) create insulating barriers between the gradient and the bulk of the fluid which reduce the effectiveness of the transfer gradient. While it is important to mix each fluid to prevent this insulating effect, any interference with the normal flow of the fluid will increase the static pressure and energy required to move that fluid through the heat exchanger, a factor which must be weighed to prevent undermining the overall efficiency of the heat exchanger.
Recognition of these principles led to the design of the counterflow heat exchanger, wherein one of the fluids is chahneled back and forth within the confines of the heat exchanger to maximize the amount of thermal energy that was transfered. Various counterflow heat exchange designs have long been practiced, the most effective and efficient exchangers placing fluids of higher temperatures within a tube surrounded by the substance to be heated, so that any heat escaping from the hot source is captured by the cold sink. While the theory behind counterflow heat exchangers has been thoroughly refined and taught to a great extent, relatively little improvement in the actual development of commercial heat exchanger designs has been seen.
One limitation found in existing configurations is that the fluid being heated makes only a single pass through, rather than flowing back and forth within, the heat exchanger.
The total counterflow heat exchanger disclosed herein recognizes this inadequacy and presents a means to pass each fluid back and forth throughout the entire heat exchange cavity by using a system of partitions surrounding an interior channel. While the total time each substance remains in the heat exchanger remains constant (because the velocity-area product remains constant for an incompressible fluid) the longitudinal path of the fluid to be heated along the transfer gradient may be increased more than threefold. Thus, a more efficient heat transfer gradient may be created over the length of the heat exchanger, thus raising the equilibrium temperature and therefore the quantum of thermal energy that is transfered to and circulated within the fluid being heated. This increases the efficiency per unit volume of the overall heat exchanger.
The total counterflow heat exchanger of this invention also enhances the even and thorough mixing of the outer fluid that is to be heated. By passing the substance back and forth within the channel, greater turbulence is produced. The increase in static pressure also increases the thermal energy of the fluid, but the longitudinal rather than lateral positioning of the partitions does not unduly impede the flow of the fluid through the interior chamber.
One might attempt to produce a total counterflow heat exchanger by spatially situating a linear tube inside a larger linear channel of equal length, and then folding those over into a counterflow arrangement within the heat exchanger cavity. However, construction of such an exchanger would present significant manufacturing problems, as well as consuming more raw materials than would make such a heat exchanger economically practical.
Furthermore, in situations where a combustion source is placed within or in conjunction with the inner tube of a heat exchanger (creating an indirect fired heat exchanger that also uses the inner tube as an exhaust stack) the construction of a tube within a reciprocal channel makes access to the combustion source for servicing or fueling very difficult.
The total counterflow heat exchanger of this invention overcomes each of these drawbacks while permitting the reciprocal flow of both fluids within the confines of the heat exchanger by employing longitudinal partitions. These partitions may be inserted in varying combinations within the cavity of the heat exchanger after the interior channel has been constructed and is in place. Access to a combustion chamber may be accomplished via a single passageway. In liquid applications, where a great multiplicity of complex channels is contemplated, the use of partitions permits far easier construction since the partitions may be inserted after the interior tubes have been constructed and positioned, rather than attempting to work with several interlocking and encapsulated channels simultaneously. Finally, because of the increased efficiency of this total counterflow heat exchanger, units with a desired thermal output may be constructed to fit into spaces where they once could not, or less fuel need be used to achieve that thermal output than was previously required.